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Tool Talk Discussion Forum

Electric motor question.

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Jay87T

03-11-2005 13:48:20




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hey guys, I think I may shop around for a bigger motor for my air compressor the pump seems really big and the motor is only 1.5 hp, question is how do you know for sure you are actualy getting the HP you are looking for in a electric motor, if its stamped 3hp on the motor is one sure it actualy is 3hp?




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George G

03-12-2005 07:42:53




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 Re: Electric motor question. in reply to Jay87T, 03-11-2005 13:48:20  
Question is, why do you think you need a bigger motor? If the compresser is running up to set PSI now,and the motor is not pulling too many amps, a bigger motor ain't going to help ya any. My compresser has a 1.5 hp motor on it. I run it at 140psi. When I got to the point that, I thought I needed more air. I picked up a second 120 gal receiver tank at a auction($25). Plumbed it in with ball valve, so I only use it when I need it. Now that I have found better ways to clean metal, (Sandblasting ain't the way to go). I don't think I have used the second tank in 5 years anyway, maybe more.

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dr.sportster

03-12-2005 04:40:53




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 Re: Electric motor question. in reply to Jay87T, 03-11-2005 13:48:20  
Stick to the quality names like Baldor and such.



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JMS/MN

03-11-2005 22:54:37




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 Re: Electric motor question. in reply to Jay87T, 03-11-2005 13:48:20  
Just remember the WAVE formula, but forget the E. W=A times V. Watts equals Amps times Volts. About 750 watts equals one horsepower. So when Sears claims their shop vac develops 4 peak HP, but it plugs into a 110 volt circuit- that is hogwash! A 15 amp circuit at 110 volts can carry 1650 watts, 20 amps at 220 volts carries a max of 4400 watts.. 5 HP on 220 draws about 3750 watts, or about 17 amps.

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Bus Driver

03-12-2005 05:18:09




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 Re: Electric motor question. in reply to JMS/MN, 03-11-2005 22:54:37  
JMS is correct about watts and horsepower- based on 100% efficiency. In the real world, efficiency is less. A true 5 HP motor on 240 volts single phase will pull about 25 amps.



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MarkB_MI

03-13-2005 04:33:29




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 Re: Electric motor question. in reply to Bus Driver, 03-12-2005 05:18:09  
Driver,

Actually, the discrepancy between volt-amps and horsepower is more due to power factor than efficiency. Power factor is the ratio between measured volt-amps and the actual power consumed by the motor.

A PF of .7 is a good rule of thumb when estimating volt-amps. So in the example you gave:

5 HP x 746 W/HP = 3730 watts

3730 / .7 = 5330 volt-amps

5330 V-A / 240 V = 22 amps.

Close enough to 25 amps. If the power factor is exactly .7, then this would work out to an 88 percent efficiency, which is still pretty low. More likely that the PF is less than .7, but that's not an easy thing to measure without the right instruments.

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dr.sportster

03-13-2005 11:19:25




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 Re: Electric motor question. in reply to MarkB_MI, 03-13-2005 04:33:29  
Ok you figured just pf Bus driver figured pf and efficiency loss for 25 amps.I think the Bus Driver has a pair of linemans pliers hanging in the back pocket and the bus full of electrical supplies.



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JMS/MN

03-12-2005 10:15:17




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 Re: Electric motor question. in reply to Bus Driver, 03-12-2005 05:18:09  
I also did the math for a 7.5 hp motor, and didn't believe it, because it came up short. I know my auger and silo unloaders run about 37 amps under rated load. The efficiency thing would explain that.



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Bus Driver

03-11-2005 16:39:42




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 Re: Electric motor question. in reply to Jay87T, 03-11-2005 13:48:20  
At 240 volts single phase, a 1 HP motor will draw about 8 amps, 1 1/2 about 10, 2 about 12 and 3 about 17. Compare this to the nameplate. Electric motors can produce almost twice the nameplate HP briefly, but they pull at least twice the amperes in the process- get hot too.



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nwb

03-11-2005 15:31:53




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 Re: Electric motor question. in reply to Jay87T, 03-11-2005 13:48:20  
Most communities will have a shop that rebuilds electric motors. Most of these will also sell new and used motors. I'd talk to one of these guys. Generally they are family shops and real good people. They'll get you the right motor for your compressor. One other thought. Electical suppliers will have an engineering booklet. In them you'll find voltages and HP rating. I think 5 HP is the top HP for 240 volts. So when places advertise 7.5 HP on a compressor you know somthing is not right. Local businessmen are the way to go. Good luck.

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GPWT

03-15-2005 09:51:37




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 Re: Electric motor question. in reply to nwb, 03-11-2005 15:31:53  
A number of manufacturers make 7.5 HP motors to run on 240V single phase. Nothing wrong with that if you have large enough conductors to safely carry the current- about 32 amps.

I think Baldor even makes a 10 HP motor for 240V single phase.



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IH2444

03-11-2005 13:56:20




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 Re: Electric motor question. in reply to Jay87T, 03-11-2005 13:48:20  
Hard to say in a simple answer. Used to be that motors has the continuous sustainable HP on them. Now many/most have a peak HP which is much higher than the sustainable HP. It seems that the peak HP might be the same as stall and smoke HP :)



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